This invention relates to a ballast, and more particularly, to an improved apparatus and method for controlling a ballast to drive various types of lamps and produce stable performance of the lamps over a large operating range.
A ballast is a device with a switched mode circuit and is often used to drive lamps, especially high intensity discharge (HID) lamps. As an example, a ballast may be implemented as a half-bridge ballast circuit 1 shown in FIG. 1. Switches 11 and 12 are turned on and off by applying appropriate gate voltages thereon for producing a stable driving voltage to the lamp or lamps 10.
The gate voltages are activated and deactivated by gate signals sent from a controller 2 which generates the gate signals and sends them to the ballast circuit 1. This is illustrated in FIG. 2, which shows a general block diagram for a ballast circuit 1. In all the circuits for a specific operation, a fixed single-mode gate signal pattern is generated at each switching cycle by the controller 2 and sent to the ballast circuit 1 for controlling the on or off state of the switches 11 and 12. Pulse width modulation (PWM) or frequency modulation has been used in ballast circuits to regulate the power delivered to the lamp. Two major pulse width modulation (PWM) patterns used are symmetric mode (FIG. 3a) and asymmetric mode (FIG. 3b). Either way, only a single mode is used. The single-mode control is simple and effective for most of the applications.
As competition increases in the market, more and more functionality and capacities are required in a single ballast circuit, such as universal line input and larger range of load. These requirements are difficult to meet using single mode control. For example, in a universal lamp dimmable ballast system, a single mode PWM control is very difficult to stabilize due to the large variations of lamp characteristics for different temperatures and lamp types. This is explained in more detail with reference to FIGS. 4a and 4b, the diagrams of the impedance lines of the ballast and the lamps.
A stable system requires only one single stable solution between the ballast lines and the lamp lines over the whole lamp operation range. However, this sometimes can not be achieved with a single mode control. As shown in FIG. 4a, the ballast lines with fixed duty cycles in symmetric mode control and two types of lamp lines are superimposed. Multiple solutions are shown with one particular duty cycle, which are marked as points A, B and C on the impedance line of lamp type T5-14W. In FIG. 4b, which shows the same case but in asymmetric mode control, two solutions, marked as points A and B on the impedance line of lamp type T5-80W, are shown with one particular duty cycle. Thus, an oscillation between the two solutions will occur.
In view of the above, there exists a need in the art for a universal ballast system that solves the instability problem as stated above so that all the lamps may perform stably over the whole lamp operation range.
The above problem of the prior art is overcome in accordance with the teachings of the present invention, which relates to a ballast system with a novel control technique. The controller for the ballast circuit generates and transmits gate signals that are of mixed modes. In particular, the gate signals are of a first mode in some switching cycles to cause the ballast circuit to produce concave ballast impedance curves, and are of a second mode in other switching cycles to cause the ballast circuit to produce convex ballast impedance curves. Combinations of these two modes results in relatively straight ballast curves, especially at a lower voltage region, whereby only a single solution exists between the ballast lines and lamp lines over the whole operation range.
Preferably, the gate signals are pulse width modulation (PWM) signals, and the first mode is symmetric mode while the second mode is asymmetric mode, which are arranged alternatively in every other one of the switching cycles.